Method for decreasing water production by gas injection in a single well operation



Aug.- 2 19. I c B. POLLOCK ET I 3,525,400 METHOD FOR DECREASING WATERPRODUCTION BY GAS INJECTION IN A SINGLE WELL OPERATION Filed Nov. 18,1968 com K C M O O NT m m mw Q Q H R Q A W HA 2 CJ 7 ud l & MB 0 w 2 mm19 VS mm WSW w m NN w mm. Hm z mm mm I Mm ATTORNEY United States Patent3,525,400 METHOD FOR DECREASING WATER PRODUC- TION BY GAS INJECTION IN ASINGLE WELL OPERATION Charles B. Pollock, Littleton, Colo., and Jack L.Shelton, Tulsa, Okla., assignors to Pan American Petroleum Corporation,Tulsa, Okla., a corporation of Delaware Filed Nov. 18, 1968, Ser. No.776,566 Int. Cl. E21b 43/25 US. Cl. 166-305 9 Claims ABSTRACT OF THEDISCLOSURE Where water is produced with crude oil, either commingledwith the oil or from a different level in the formation, it has beenfound that water production rates can be substantially lessened byinjection of a gas having a relatively high preferential solubility inoil into the formation. Under these conditions a trapped gas saturationis developed inthe water, reducing its mobility and thereby bringingabout a substantial decrease in the fiow thereof to the well bore. Thiseffect is generally only temporary lasting for several months. Afterwater production again approaches pre-gas injection levels the proceduremay be repeated.

INTRODUCT ION The present invention relates to an improved method of oilproduction and more particularly is concerned with a method for reducingthe flow of water which is either commingled with the oil or is producedfrom a separate zone.

BACKGROUND Production of water along with crude oil is a problem thatgenerally is tolerated so long as the lifting and separating costs donot become uneconomical. Where water entry occurs largely in a level ofthe formation different from that yielding the oil, the water flow isoftentimes controlled by squeeze cementing or casing the producingformation and perforating through only to the oil production zone. Thesemethods while generally effective have not necessarily met withunqualified success and of course cannot be used with any positiveefiect where the oil and water are commingled and are producedessentially from the same zone.

BRIEF DESORIPTION OF THE INVENTION In the accompanying drawing thesolubility of light hydrocarbon gases in oils of different gravities andwater, as affected by pressure, are shown.

We have now discovered a method that is not only selective for thereduction of the water flow'whether commingled with or essentiallyseparate from the oil producing zone(s)but which lowers the oilviscosity substantially thus increasing the oil producing rate. Briefly,this is accomplished by first discontinuing production from the well andthen injecting into the formation a gas or gaseous mixture having asubstantially higher solubility in oil than in water. Under theseconditions a trapped gas saturation is created in the water thereby decreasing the relative permeability of the formation to water and,accordingly, resulting in decreased water production rates. On the otherhand, the injected gas or gaseous mixture which is highly soluble in theoil goes into solution causing a marked reduction in viscosity. Thus, itis seen that by such injection the mobility of the oil is increasedwhile that of the water is decreased resulting in a marked change in therelative oil and water production rates.

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gas should be injected, e.g., at least about 100,000 cubic feet per footof net pay involved in order to establish a large trapped gas saturationin the water-containing region of the formation. When the well is putback on production the water rate is reduced because of the resistanceto flow due to the trapped gas.

The process of our invention is more effective the greater the degree ofseparation of the oil and water producing levels in the formation. Gastrapped in the water can diffuse into the oil and be absorbed eventuallydepleting the gas saturation in the water in which case the cycle isrepeated by halting production and again injecting gas into theformation. Although best results are usually obtained when the oil andwater producing zones are separated, benefits can also be secured whenoil and water are produced from the same strata. This is particularlytrue in viscous oil reservoirs where channeling and bypassing of oil byinjected or natural water results in localized regions of high watersaturation.

The choice of the gas depends mainly on the properties of the crude oiland the pressure. Many undersaturated crudes have a high solubility formethane and in such cases a dry field gas can be used. By the expressiondry field gas we mean one which is principally methane, i.e., a minimumof 90-95% methane and the balance being predominantly ethane. Usually,however, a mixture of methane with ethane, propane and/or butane, etc.,is ordinarily preferable to insure good solubility. This is especiallytrue with reservoirs having pressures of less than 600 psi. In no caseis it necessary to use a gas which exceeds the requirements formiscibility with the reservoir oil. Generally speaking the compositionof the gas injected may vary rather widely. For example, in the case ofmethane-propane mixtures we have found that 20 mol percent of propanemay be used to advantage with asphalt-containing crudes. With nonasphaltic crudes the proportion of propane may be as high as 25 molpercent. The injected gas, however, in order to be effective should beat least 13 times more soluble in the reservoir oil than in water.

The effects of oil gravity and pressure on the solubility of methane orpropane in such oil as well as in water are ShOIWIl in the accompanyingdrawing. This series of fluids illustrates the influence of oil gravity,pressure and composition of injected gas on the relative solubility ofsuch gas in both oil and water. Thus it is seen that the relativesolubility is greater for the higher API gravity oils and the 16 APIgravity oils represents the approximate lower limit of applicability ofour process although it should be noted in this connection that theexample hereinafter described shows that the process of our inventioncan be employed successfully where the API gravity of the oil is notmore than about 14. Further the curves demonstrate that as the oilgravity increases, the solubility of gas therein becomes greater. Also,it is seen that with any given oil the solubility of methane, propane orequivalents thereof in such oil can be substantially increased by theaddition of one of the lighter hydrocarbons such as propane or butane.The lower most curve based on 16 AJPI gravity oil illustrates theapproximate minimum relative solubility of methane, i.e., a ratio of13:1 which will bring about a distinct decrease in water mobility withno apparent loss of oil mobility. In fact, as previously stated, oilmobility can be increased particularly where hydrocarbons such aspropane and/or butane are present.

The process of our invention will be further illustrated by means of thefollowing specific example.

EXAMPLE The well in which our invention was employed had To accomplishthis result, relatively large volumes of about feet of net pay beginningat a depth of 1220 feet. The formation had a porosity of 24%, a watersaturation of 25%, and a permeability of about 640 millidarcies. Theoriginal displacement mechanism in the field in which this well waslocated was by Water drive. The oil had an API gravity of 14.4", aviscosity of 1,000 centipoises and contained little or no dissolved gas.Oil production amounted to about 25 bbls./day. Water production hadhistorically been 40-50% of the producing well fluids. Production wasdiscontinued and thereafter for a period of two weeks 11,700,000 cubicfeet of gas consisting of about 85% methane and 15% propane was injectedinto the pay zone of this well. Thereafter, injection of gas wasdiscontinued and the well placed on production. Increased oil productionrates were obtainedwith an average during the first month of about 75bbls./ day-as a result of lowering the viscosity of the oil to about 625centipoises caused primarily from solution of propane into the crude. Avery significant decrease in water production also resulted. The watercut after resumption of production was less than Over the next twomonths the water content rose slowly to a value of about 12% The averageoil production during this period was about 40 bbls./day.

The data appearing in the tables below were calculated using StandingsCorrelation of properties of natural hydrocarbon mixtures of gas andliquid (Volumetric and Phase Behavior of Oil Feld Hydrocarbon Systems,Reinhold Publishing Co., New York, 1952). The data shown for watersolubility were based on the original data of Vapor- Liquid Elqiuibriumfor Binary Hydrocarbon-Water Systems, by Riki Kobazashi and Donald L.Katz, Industrial and Engineering Chemistry, 45, p. 440 (1953).

Table I gives the volume of methane and propane dissolved in a 16 APIstock tank oil at 80" F. and bubble point pressure between 200 and 2000p.s.i.a. In addition the incremental volume of methane or propanedissolved above the base pressure of 200 lbs. is given.

Table II gives the same information as Table I except that a 26 APIstock tank oil is considered.

Table III tabulates the cubic feet of a hydrocarbon gas such as methaneor propane that is dissolved in water in the pressure range of 200-2000lbs. at 90 F. This table also gives the incremental volume of gas thatis dissolved in water above a base pressure of 200 lbs.

Table IV takes the incremental volume figures or numbers of Tables I-IIIand ratios the incremental volumes of methane and propane to theincremental volume of Water dissolved in the particular API stock tankoil crude at the designated pressures. This is the information on whichthe curves on the accompanying drawings are based.

TABLE I MCF of either methane or propane that a 16 API stock tank oilwill dissolve less the amount that will dissolve at 200 Total MCF ofeither methane or propane that a 16 API stock Bubble point tank oil willdissolve p.s.i.a. pressure, p.s.i.a. Methane Propane Methane PropaneTABLE II MCF of either methane or Total MOF of either propane that a 26API methane or propane that stock tank oil will dlssolve a 26 API Stocktank oil less the amount that w1ll Bubble point will dissolve I dissolveat 200 p.s.1.a. pressure, p.s.i.a. Methane Propane Methane Propane 20 320 0 45 68 25 36 71 110 51 78 140 210 120 178 200 300 180 268 275 410 255378 TABLE III MCF of gas that water Total MCF of gas will dissolve lessthe Bubble point (methane or propane) amount that will pressure,p.s.i.a. that water will dissolve dissolve at 200 p.s.i.a..

TAB LE IV Ratio of additional gas the oil will dissolve to theadditional gas the water will dissolve Bubble point 16 API 26 APIMethane Propane Methane Propane From the foregoing description andexample it will be seen that the process of our invention is elfectiveto bring about a distinct reduction in water flow coming from an oilbearing formation. Such flow may be either commingled with the oil orfrom a separate level in the same formation in which the oil is present.Thus, it is seen that by our invention we are able to improvesubstantially the efficiency of a single producing well having waterinflux problems, merely by the periodic injection of a gas or gasmixture having a preferential solubility in oil at least about 13 timesas great as in water. After suflicient gas has been injected the well isplaced back on production to realize a sustained decrease in waterproduction. Whenever the water cut increases to the point whereeflicient operation of the well becomes difficult, the above cycleshould be repeated.

We claim:

1. In a process for recovering oil from an oil-bearing formation havinga production well extending into said formation and wherein asubstantial water flow from said formation accompanies the flow of oilinto said well, the method of substantially reducing said water flow byinjecting into said formation hydrocarbon gas which is at least about 13times more soluble in said oil than in water under the conditions of thereservoir to develop a trapped gas saturation in said formation waterthereby decreasing the relative permeability thereof to water, wherebythe oil-water ratio produced into said well is substantially greaterthan prior to said injection step and thereafter producing fluids fromsaid well.

2. The method of claim 1 in which the gas injected consists of at leastabout 75 mol. percent methane.

3. The method of claim 1 in which the crude oil involved is asubstantially non-asphaltic crude and wherein the propane content of theinjected gas is not more than about 25 mol. percent.

4. The method of claim 1 wherein the cycle of steps outlined is repeatedafter water production approaches the water production rate prior totreatment.

5. The method of claim 1 wherein the API gravity of the oil produced isnot less than about 14.

6. The method of claim 1 wherein the amount of gas injected correspondsto at least about 100,000 cubic feet per foot of net pay.

7. The method of claim 1 wherein the oil involved is an asphaltic crudeand the propane content of the injected gas is not more than about 20mol. percent.

8. The method of claim 1 wherein the oil involved is an undersaturatedcrude and the hydrocarbon gas is a dry field gas.

6 9. The method of claim 1 wherein the water flow is 2,909,224 10/1959Allen 166-305 commingled Water. 3,252,512 5/1966 Baker et a1. 166-305 X3,283,818 11/1966 Santourian 166-305 References Cited UNITED STATES A S5 ERNEST R. PURSER, Primary Examiner 2,788,855 4/1957 Peterson 166-306 XI. A. CALVERT, Assistant Examiner UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,525,h00 Dated Au ust 25, 1970Inventor) Charles B. Pollock and Jack L. Shelton It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, line 27, "Feld" should read --Field--; line 30, "Elqiuibrium"should read --Equilibrium"; line &2, "90" should read --80--.

Column h, in TABLE IV, below "Bubble point" insert -pressure,p.s.i.a.--.

SIGNED ANu SEALED Atlmt:

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